This invention relates to a hydraulic snubber and more particularly, to a hydraulic snubber especially advantageous in connecting the head assembly of a grapple to the boom mount such as are found on the rear frame of crawler or rubber-tired tractor skidders.
A grapple with which the invention can be used advantageously is seen in my earlier U.S. Pat. No. 4,358,147 and a skidding grapple is seen in co-owned U.S. Pat. No. 4,400,132.
Uneven ground and rapid changes in speed and direction of the skidder cause the empty grapple head assembly to swing violently when unrestrained. This swinging motion causes the grapple head to bang into the boom and rear frame of the skidder, resulting in damage to the boom, grapple and tractor.
The purpose of the snubber, which is the connecting link between the grapple head and boom, is to dampen the violent damage-causing motion out. At the same time, the snubber link, being a universal type joint, must allow controlled movement of up to 90.degree. each side of plumb in the lateral and longitudinal direction relative to the tractor.
Current means of dampening this motion are subject to high maintenance and frequent adjustments in order to perform well with consistency and are rendered useless when contaminated with oil or grease. The inventive snubber described here requires no adjustment and infrequent maintenance. It is a sealed unit, has only two moving parts, and works on the known principle of force induced fluid displacement (see U.S. Pat. No. 3,592,503). Grease or other high viscosity synthetic lubricant is contained in two cylindrical chambers, divided into two inversely variable compartments.
Two diametrically opposed vanes cause the chamber division. One vane is integral with a rotatable housing, and the other with a rotor which is in turn keyed to a fixed pin (or the opposite may be the case). Chamber compartment volumes vary inversely as one vane is rotated relative to the other. This variation in volume causes the dampening medium to be forced from one compartment to the other across the vanes. The passage or area through which the dampening medium must cross is such that it restricts free flow and creates a pressure drop across the vanes so that free rotation is restricted. This effect is true with rotation in either direction.
According to the invention of my earlier application, a novel construction of snubber was provided which made possible universal joint action and further a vane extension configuration was provided that adjustably responded to pressure changes so as to achieve a desired snubbing or shock absorbing action.
During further tests the resilient vane tip/orifice pocket concept has proven to work quite well to dampen the swing of an empty grapple which is the purpose for which it was intended. In other words, the inertia of the empty grapple in motion is effectively controlled by the compensating action of the pressure induced variable orifice concept in the resilient vane.
These same tests also indicate that this system by itself has certain limitations when subjected to two extreme conditions: (1) the higher rotational inertia generated by a loaded grapple and (2) higher fluid viscosity caused by low ambient temperatures, and, of course, the two combined. The real problem caused by these extremes is pressure buildup in the chamber or chambers of the snubber as resistance to rotation is accomplished. Pressures in excess of 5,000 psi have been developed in laboratory and it is estimated that under actual working conditions in the field pressures of 8,000-10,000 psi may be possible.
1. Ambient temperature conditions: The combination of resilient vane material, orifice/pocket design and fluid viscosity based upon extensive tests, performed adequately in an ambient temperature range of between plus 30.degree. to plus 90.degree. F.
As the temperature falls below 30.degree. F., the viscosity of the fluid increases and the flow is further inhibited through the vane tip orifices. This causes increased resistance to rotation beyond what is required and pressure rises in the snubber chamber to the point where sealing becomes a problem.
Resilient vane material and/or inherent fluid viscosity can be altered but still will only accomodate a relatively narrow temperature range. In many actual working conditions this would involve the necessity of changing one of these elements at least two or three times a year. In some cases such as Eastern Oregon and Washington where temperatures vary from -30.degree. in the morning to +90.degree. in the afternoon, this would have to be done twice a day. This, of course, is not acceptable and the snubber must be able to handle as broad a range of temperatures as possible with minimal adjustment to the components in the snubber.
2. High rotational inertia generated by a loaded grapple: This rotational inertia is directly related to the load in the grapple and the angular position of the center of gravity of the loaded grapple when it is off the suspended vertical. An empty grapple, inhibited by the snubber, will generally swing only a few degrees depending upon how tightly it is snubbed and the external forces acting upon it.
This, however, is not the case with a loaded grapple which does not really need a snubber to keep it from banging into the boom and tractor. As a loaded grapple is forced to rapidly change position behind the tractor due to rough terrain, stumps, trees, log decks, etc., the relative positions of the vanes in the snubber are forced to change at the same rate. Frequently, for example, the change can be 90.degree. or more and can be made in one second or less.
In these extreme conditions, a large volume of snubbing fluid is forced through the orifices across the vanes in a very short period of time. This can build up very high pressures in the snubber chambers which is another source of seal problems.
Here again, the unit must be capable of accomodating these extreme conditions in a trouble-free manner and, in this case, there are no seasonal or daily adjustments that can be made to alleviate the problem.
It can be seen that a combination of these conditions would be even more aggravating to the seal problem. This complicated problem has been solved through the use of a valved bypass around the stationary vane.
Other objects and advantages of the invention may be seen in the details of the ensuing specification.